Different particle systems impact performance differently.
If you are using particles extensively, upgrade the video card to an ATI x800 or nVidia 6800 or better.
The 9800 card can be unbearably slow with particles, almost as bad as the 8600 in my Powerbook.
One thing you can try is changing the emitter parameters to make it less taxing on the system.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com